Switching power supplies and DC/DC converters have currently been employed in many of power supplies of electronic devices. Known as a capacitor used in these power supplies is a power supply bypass capacitor.
For the power supply bypass capacitor, an aluminum electrolytic capacitor or tantalum electrolytic capacitor having a large capacity or a laminated ceramic capacitor having a low capacity has been used according to circuit parameters such as its power capacity and switching frequency and a smoothing coil employed.
Though the electrolytic capacitors mentioned above have excellent advantages in that they can easily attain a large capacity as a power supply bypass (smoothing) capacitor, they are problematic in that they are large in size; they are poor in low-temperature characteristics; they arouse a fear of short-circuiting; their internal impedance is relatively high, so that their dielectric dissipation factor (tan δ) due to equivalent series resistance (hereinafter referred to as “ESR”) is high, whereby loss occurs steadily, which generates heat; they fail to attain sufficient frequency characteristics, so that their smoothness is insufficient; etc.
On the other hand, technical innovations in recent years have advanced thin-layer forming techniques and lamination techniques for dielectric layers and internal electrodes, whereby the electrostatic capacity of laminated ceramic capacitors has been approaching that of electrolytic capacitors. Therefore, proposals to replace electrolytic capacitors with laminated ceramic capacitors have been under study.
In power supply bypass capacitors, it is important to reduce ripple noise in order to attain a sufficient smoothing action. The ripple noise can be suppressed if the ESR of the capacitor is reduced.
It is therefore preferred that capacitors having a low ESR be used in power supply bypass circuits, whereby proposals to use laminated ceramic capacitors having a low ESR in power supply circuits have been under study.
When a laminated ceramic capacitor having a low ESR (e.g., a laminated ceramic capacitor having an ESR of less than 10 mΩ) is used in a power supply circuit, however, the ESR of the smoothing circuit greatly influences phase characteristics of a feedback loop in secondary circuits such as a DC/DC converter having a feedback circuit to become a power supply and a switching power supply, in particular, the ESR may become extremely low, whereby it has been impossible to attain sufficient reliability.
Namely, when a laminated ceramic capacitor having a low ESR is employed as a power supply circuit bypass (smoothing) capacitor, the secondary smoothing circuit is equivalently constituted by L and C components alone, so that only ±90 degrees and 0 degree exist as phase components existing in the circuit, and phases have no room, whereby oscillations are easy to occur. This oscillation phenomenon has also occurred when load fluctuates in a power supply circuit using a three-terminal regulator.
For overcoming the problems mentioned above, it has been studied to add a resistance component having such a level that the oscillation phenomenon can fully be prevented from occurring to a laminated ceramic capacity having a low ESR. For example, a so-called CR composite electronic component has been proposed. For instance, Japanese Patent Application Laid-Open No. HEI 8-45784 proposes an electronic component in which an end part of a laminated ceramic capacitor is turned into a semiconductor by using a carbide and a reducing agent.
For example, Japanese Patent Application Laid-Open No. SHO 59-225509 discloses one in which a resistive paste such as ruthenium oxide is laminated on a laminated ceramic capacitor, and they are baked at the same time, so as to yield a resistive element.
Japanese Patent Publication No. 2578264 proposes a CR composite component in which a metal oxide film is provided on a surface of an external electrode, in order to attain a desirable ESR.
Also proposed is a laminated ceramic capacitor using a conductive resin as a constitute material of an external electrode. In this case, the conductive resin contains spherical or scaly particles made of a metal such as silver. The ESR of this type of smoothing capacitor (laminated chip capacitor) is relatively low in general, i.e., 10 mΩ or less. Therefore, in order for the ESR of the external electrode to rise to at least 40 mΩ, which is said to be able to sufficiently prevent the above-mentioned oscillation phenomenon from occurring, the content of the conductive material is required to be as small as possible, for which studies have been under way.